CDMA systems are interference-limited systems. For example, the IS95/CDMA2000 standard seeks to minimize interference on reverse links (i.e., access channels) during call setup through the use of low power access probes. Additionally, the standard seeks to minimize interference during calls by using closed loop power control techniques.
For wireless telephone calls, the base station (BS) measures signal strength (E/I) received from the mobile subscriber (MS) and sends an up/down power adjustment instruction via a power control sub-channel (i.e., dedicated bits in a forward link frame). In turn, the MS measures frame error rate (FER), on the forward link, and reports these measurements to the BS using Power Measurement Report messages. In response, the BS adjusts its forward power.
In contrast to telephone calls, dispatch (or group) calls typically involve several MSs, all participating in the call. In a dispatch call, one MS is transmitting (i.e., in talk mode) while the other members of the group are receiving (i.e., in listening mode) at any given time. When the MS with the transmit prerogative releases its “Push-to-talk” (PTT), a message is sent to the BS, and the BS places the traffic channel in an “open mode.” While in this mode, any other MS can request a transmit prerogative by signaling a PTT. The BS allocates a transmit prerogative to the first arriving request. While this description is for a single-cell call, it can be applied to a multi-cell call with the addition of a central dispatch call controller, which arbitrates transmit prerogative requests instead of a controlling BS.
In a CDMA system such as that described in U.S. Pat. No. 6,519,239 (U.S. patent application Ser. No. 09/443,538), entitled “METHOD AND APPARATUS FOR PROVIDING DISPATCH SERVICE IN A CDMA COMMUNICATION SYSTEM,” group call voice frames are transmitted using standard CDMA traffic channel procedures except that a common long code is used instead of an individual long code computed from the MS's ESN. Thus, the MSs in the group call share a single traffic channel. However, in such a system, listening mode MSs cannot request forward power adjustment by the BS. As a result, distant listening MSs, which require greater forward power from the BS, may experience excessive frame error rates and unacceptable voice quality. Therefore, a need exists for an apparatus and method to provide power control for listening participants of CDMA dispatch services.